1. Field of the Invention
The present invention relates to a method for manufacturing a semiconductor light emitting device.
2. Background Art
A GaN-based mixed crystal such as AlGaInN and the like has a large bandgap of a direct transition type, and is used as a material for light emitting devices (for example, LED: light emitting diode) of short wavelength.
Currently, since there is no good substrate which is lattice-matched with a GaN-based mixed crystal, a method is used for convenience that grows a GaN-based mixed crystal on a sapphire substrate via a low-temperature-grown amorphous layer or a polycrystal-form buffer layer.
At this time, nearly half of the emitted light is reflected at the interface between the sapphire and the GaN-based mixed crystal because there is a difference in refractive index between the sapphire and the GaN-based mixed crystal, causing a decrease in efficiency. Accordingly, a method in which the substrate is separated by an irradiation of a high-power laser with a short wavelength from the sapphire side to decompose the GaN facing the substrate is tried.
However, this method causes damage of the device structure unit due to heat and/or stress during the laser irradiation, and therefore decreases efficiency and/or generates high-density threading dislocations due to lattice mismatching, leading to degradation of device characteristics. Further, a crack easily occurs because of residual strain, causing a decrease in yield.
These problems are significant particularly for an ultraviolet region of wavelengths shorter than 400 nm (nanometers) in which crystal defects often influence efficiency, even in a region of wavelengths longer than 370 nm in which the light is not so absorbed by GaN.
Japan Patent No. 3803606 discloses a method that forms a thermal-diffusion control layer having a lower thermal conductivity than a group-III nitride semiconductor which forms a first semiconductor layer, and irradiates light beam which is absorbed in the first semiconductor layer to decompose the first semiconductor layer.